Sulfuric acid continues to be the largest volume inorganic acid in use today and is generally cosidered to be the most important industrial chemical. Many metallic and nonmetallic substances resist various concentrations of sulfuric acid either as relatively pure acid-water solutions or in combination with other acids, contaminating ions, dissolved gases or solid particulate matter. Also, the selection of a sulfuric-acid resistant material will often have to be altered depending upon whether the acid to be encountered will be a still liquid or in a condition of low or high velocity flow. Also, the corrosiveness of sulfuric acid depends very much upon temperature and concentration. Hot strong acid presents the greatest problem. This is true for oleum (up to 30% excess sulfur trioxide) as well.
While both cast and wrought alloys have been developed to handle various strengths of sulfuric acid, the wrought alloys are generally inferior to the best cast alloys in resistance to hot, concentrated acid. This situation results primarily because such cast alloys usually contain high levels of silicon. On the other hand, the casting alloys, because of their high levels of silicon, are quite difficult to weld and machine, let alone hot or cold form into items such as sheets or tubes.
Nevertheless, the use of silicon as an addition element remains attractive because of its wide availability and very low cost coupled with its pronounced tendency to improve the resistance of these alloys to hot, concentrated sulfuric acid. However, silicon tends to embrittle sulfuric acid resistant alloys when present in concentrations above about 2-2.5% by weight. The ductility and fabricability of such acid resistant alloys are also reduced by increasing the amount of Cr, Mo, Cb and even Ni, although Si has the most detrimental effect. Furthermore, the reduction in ductility from using various elements is cumulative.
One of the earliest prior art alloys said to be useful for applications requiring resistance to sulfuric acid is found in Payson, U.S. Pat. No. 2,251,163, which dealt with nominally Fe--12% Ni--24% Cr--2% Mo--1.2% Cu alloys but claimed ranges of 20 to 28% Cr, 0.5 to 5% Mo, 0.5 to 5% Cu, up to 1.25% Si and 10 to 16% Ni. Payson's alloys were low in both Si and Ni content and of very inferior corrosion resistance to sulfuric acid to today's standards.
Later, Sullivan U.S. Pat. No. 2,423,665, July, 1947, disclosed alloys nominally having an element content of Fe--28% Ni--20% Cr--1.7% Mo--2.5% Cu--2.5% Si--0.75% Mn, but claiming 27-30% Ni, 19-22% Cr, 0.5-2% Mo, 0.7-3.5% Si, 1.25-3.5% Cu and 0.5-1% Mn. The Sullivan patent places stress on the fact that Ni content is always greater than Cr content in that invention. Sullivan also states that it is preferred to keep Mn content close to the mean, or about 0.75%. Sullivan further states that reducing Mo content below 1% greatly increases the rate of corrosion of his alloys by sulfuric acid.
There have been many other attempts to develop alloys resistant to sulfuric acid, especially hot concentrated sulfuric acid, which have improved ductility but which retain their acid resistance. Parsons, U.S. Pat. No. 2,185,987, January, 1940, disclosed what has come to be known as Alloy 20, nominally Fe--25% Ni--20% Cr--2.5% Mo--3.3% Cu, for resistance to sulfuric acid. This alloy has undergone a few changes over the years, and Scharfstein, U.S. Pat. No. 3,168,397, February, 1965, disclosed a similar alloy now called 20Cb3, but with Ni content increased to 34% and the addition of Cb or Ti. Both of these alloys along with a number of similar highly-modified stainless steels are available in all of the wrought forms but are limited to use at 60.degree.-65.degree. C. with concentrated sulfuric acid.
Moskowitz et al, U.S. Re. 27,226, November 1971, is directed to alloys having broad ranges of Ni, Cr and Mn, and also provides for optional amounts of Si, Mo and Cu along with 0.25 to 0.45% S and up to 0.5% P. Aside from the fact that Si, Mo and Cu are optional, these sulfur contents are quite intolerable for hot sulfuric acid resistance, and phosphorus present over about 0.05% maximum lowers notch ductility in austenitic stainless steels and also tends to harden them considerably by a precipitation-hardening mechanism.
Goda, U.S. Pat. No. 3,811,875, May 1974, discloses alloys containing Ni, Cr, Mn and Cu and also requires 0.25 to 2% A1 and 0.15 to 0.75% sulfur and selenium to improve machinability. Up to 3% Si and up to 3.5% Mo, are also allowed which makes both elements optional. Goda also states that all or part of the Mo may be replaced by W. The S and Se additions are the heart of the Goda invention and included to improve their free-machining properties. While they tend to do so, they are detrimental to weldability, formability and corrosion resistance.
In U.S. Pat. No. 2,938 787, May 1960, Boyd, et al., disclosed a casting alloy of excellent resistance for its time to hot concentrated sulfuric acid. It is still marketed under the name of Illium B. In U.S. Pat. No. 3,008,822, Nov. 14, 1961, the same inventors disclosed a low Si version of the same alloy with considerable improvement in machinability and weldability but at the expense of greatly reduced resistance to the hot, concentrated acid. It is marketed as Illium 98 for use in up to 98% sulfuric acid but is only used as a casting alloy.
More recently Yamaguchi, et al., U.S. Pat. No. 4,141,767, Feb. 1979, disclosed a two-phase stainless steel containing 10-75% ferrite and broad ranges of Ni and Cr. Yamaguchi also provides for optional Mo, Cu, Mn, Si and Cb contents as well as 0.06 to 6% Al.
Another problem with such alloys of the prior art is that the compositional ranges include extensive secondary or multiple matrix phases such as sigma, chi, alpha-prime and gamma-prime. There are many applications in which duplex alloys, of nearly equal matrix division between austenite and ferrite, provide excellent corrosion resistance. However, hot, concentrated sulfuric acid is not one of those situations.
Abo, et al. U.S. Pat. No. 4,172,716, Oct. 1979, also makes claim to broad element ranges for Ni, Cr and Mn in alloys said to resist pitting corrosion. Abo further provides for 0.1 to 6% Si, which is equivalent to making the Si content optional, inasmuch as it is virtually impossible to produce such alloys by ordinary means and with raw materials that would result in less than 0.1% Si. In similar manner, Abo's range of Mo of from 0.01 to 6% amounts to an optional content of this element. One or both of the elements Cu (0.1 to 4%) and Cb (0.1 to 2%) may be optionally includes. A similar Japanese patent, 58-210157, issued Dec. 7, 1983 and assigned to Sumimoto Metal Ind. discloses an alloy for oil-well piping and has broad ranges for Ni,Cr, Cu, Cb and Mo plus 1/2 W of 1.5% to 4%. It also provides for 0.10 to 0.25% C and 0-1% Si.
The patnet to Kudo, et al., U.S. Pat. No. 4,400,349, Aug. 1983, similarly discloses alloys for oil-well casing, tubing and pipes with broad ranges of Ni, Cr and Mn but require less than 1% Si and provide optionally for 0 to 12% Mo and 0 to 2% Cu.
Other prior art wrought metals and alloys also have severe limitations with respect to handling hot, concentrated sulfuric acid. Zirconium metal is available in all wrought forms but quite expensive and only suitable in hot sulfuric acid concentrations below 65-70%. Unalloyed tantalum has resistance to sulfuric acid solutions over the entire range of concentrations up to about 98% acid strength and temperatures up to about 230.degree. C. However, tantalum is extremely scarce and expensive.
Ordinary carbon steel has long been used in handling sulfuric acid at ambient temperatures in the concentration range of 65-100% under static and low-velocity conditions. At temperatures above 25.degree. C., however, attack by the acid may become erratic and catastrophic.
At ambient temperatures, austenitic stainless steels, for example type 304, exhibit passivity in sulfuric acid above 93% concentration. Mo extends the passive region to as low as 90% at ambient temperatures. At higher temperatures passivity is extended to concentrations above about 98.5-99%. Nevertheless, care must be taken when using stainless steels in the 98% to 100% concentrations at high temperatures; velocity conditions, reductions in acid concentration, or change in oxidant levels may initiate very high corrosion rates.
Austenitic high Si stainless steels have recently been developed that provide remarkable resistance to nitric acid above 95%. The cast version has a typical composition of Fe--21% Cr--16% Ni--5% Si--0.02% C. The wrought version of this alloy, designated A-711, has a typical composition of Fe--18% Cr--18% Ni--5.3% Si--0.02% C. The A-611 alloy also has useful corrosion resistance to 99% sulfuric acid up to 120.degree. C.
In spite of all these efforts there still remains a need for alloys which not only have good resistance to hot concentrated sulfuric acid but which also have good ductility allowing them to be used in the manufacture of various items of commerce where wrought alloys are required.